Revised and resubmitted to Quaternary International on 21 June 2015 Formal subdivision of the Quaternary System/Period: past, present, and future. Martin J. Head a,*, Philip L. Gibbard b a Department of Earth Sciences, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada b Cambridge Quaternary, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK * Corresponding author E-mail address: [email protected] (M. J. Head) Key words: Quaternary, Pleistocene, Holocene, Anthropocene, GSSP ABSTRACT The Quaternary System/Period represents the past 2.58 million years and is officially subdivided into the Pleistocene and Holocene series/epochs, with the base of the Holocene assigned an age of 11,700 calendar years before AD 2000. The two lowest stages of the Pleistocene, the Gelasian (base 2.58 Ma) and the Calabrian (base 1.80 Ma), have been ratified and these effectively constitute the Lower Pleistocene Subseries. All other official subdivisions are pending. For the Middle Pleistocene, three candidate global boundary stratotype sections and points (GSSPs) are under consideration: the Valle di Manche in Calabria and Montalbano Jonico in Basilicata, both in southern Italy, and the Chiba section in Japan. The Matuyama–Brunhes Chron boundary (~773 ka) serves as the principal guide for the base of the Middle Pleistocene. The base of the Upper Pleistocene is generally agreed coincide approximately with that of the last interglacial (Marine Isotope Substage 5e ~130 ka): the Fronte section near Taranto in southern Italy represents a possible candidate GSSP, but an Antarctic ice core might also serve this purpose. Another important task is the subdivision of the Holocene for which a tripartite scheme, with boundaries at 8200 and 4200 years B.P., has been suggested. Additional fine-scale formal subdivision of the Quaternary is being explored, with the Last Glacial Maximum serving as a good example. The “Anthropocene” is both an informal and undefined interval of time that includes the present day. Its duration, formal/informal status, rank, and method of definition are all under debate, with proponents suggest that it be defined as a formal unit beginning with the world’s first nuclear bomb explosion, on July 16th 1945. Suggested and proposed GSSPs are compared and critiqued. The history leading to the ratification of the Quaternary Period in 2009 is examined drawing upon published and unpublished material. 1 1. Introduction The Quaternary Period/System represents the past 2.58 Ma and is officially subdivided into the Pleistocene and Holocene series/epochs. The Quaternary Period/System, Pleistocene Series/Epoch, and Gelasian Stage/Age are all defined by a global boundary stratotype section and point (GSSP) at Monte San Nicola in Sicily (Gibbard and Head, 2010; Gibbard et al., 2010; Fig. 1). The Pleistocene is traditionally subdivided into Lower, Middle, and Upper Pleistocene subseries/subepochs, and although these subdivisions are not yet officially defined, the Calabrian Stage has been defined by the Vrica GSSP in Calabria, southern Italy, effectively completing the definition of the Lower Pleistocene (Cita et al., 2012). The Holocene is defined by a GSSP in the Greenland NGRIP ice core (Walker et al., 2009). No other GSSPs have yet been designated for the Quaternary. The current official time scale for the Quaternary, as sanctioned by the International Union of Geological Sciences (IUGS), including both ratified and suggested and proposed GSSPs, is shown in Figure 2. We draw a distinction between the terms “formal” and “official” with respect to chronostratigraphic units. Here, “formal” refers to a unit whose boundaries are widely understood and accepted, whereas “official” refers to a unit whose lower boundary is defined by a GSSP proposed by the International Commission on Stratigraphy (ICS) and ratified by the Executive Committee (EC) of the IUGS. In this sense the Lower, Middle and Upper Pleistocene subseries are formal but not official units; the Holocene Series is both. The objective is for all major boundaries of the Quaternary System to be defined by GSSP. This requires recommendations to pass up through a “chain of command”. The initiative begins with the Subcomission on Quaternary Stratigraphy (SQS), one of currently 16 constituent subcommissions of the ICS. The SQS establishes an international boundary working group (officially an ICS “task group”), led by a convener, and tasked with proposing a GSSP. This task itself is preceded by a decision by the working group on the primary guide to be used for the boundary. The primary guide might be a paleomagnetic boundary, or a geochemical excursion, or a paleontological event, or some other marker that can be recognized widely, and its selection will determine the appropriateness of a candidate GSSP, which is why such a decision is taken first (the principle of “correlation precedes definition”; Cowie, 1986; Cowie et al., 1986; Remane et al., 1996). The boundary must satisfy several criteria. As a first priority, it must facilitate global correlation. It should also reflect the narrative of Earth history by separating one chapter in that story from another, while respecting historical usage as far as is reasonable. Once the primary guide has been decided, the best stratotype is sought. In reality, multiple criteria are used to recognize a boundary, and a candidate GSSP ideally will display these in a well exposed, expanded, and accessible section. The full range of ideal criteria is listed in Table 1. A decision on whether to recommend a particular candidate GSSP is made first by the relevant boundary working group (officially a “task group”), next the SQS, and subsequenly the full voting membership of the ICS, this being the ICS executive and the chairs of the constituent subcommissions. A 60% supermajority is needed for a GSSP proposal to proceed to the next level, and at any level the proposal can be returned for clarification or revision. 2 Once approved by ICS, a proposal is forwarded to the executive committee of the IUGS for ratification, which is granted only after final scrutiny especially to ensure that correct procedures have been followed. Immediately ratification is granted, the candidate GSSP has full legitimacy as a chronostratigraphic standard. In spite of steady progress achieved over the past seven years in subdividing the Quaternary, much work remains to be done. This includes defining the Middle and Upper Pleistocene, subdividing the Holocene, and resolving the issue of the Anthropocene. Consequently, it seemed timely to organize a special session on “The Quaternary System and its formal subdivision” at the First International Congress on Stratigraphy, STRATI 2013, held in Lisbon, Portugal, during 1–7 July, 2013 (Rocha et al. (2013). Most of the papers presented are included in this special volume. The full-day session was sponsored by the ICS Subcommission on Quaternary Stratigraphy (SQS), which is charged with recommending Quaternary GSSPs to the ICS, and the Stratigraphy and Chronology Commission of INQUA (SACCOM). It was convened by Martin J. Head (Chair, SQS), Philip L. Gibbard (President, SACCOM), and Thijs van Kolfschoten (Secretary, SACCOM). Further details are given in the guest editorial (Head et al., this volume). The present paper documents the fraught and tortuous history of the Quaternary as an official period/system (below, Table 2, and Appendices 1 and 2), comprehensively reviews its formal subdivision, and examines the tasks and challenges that lie ahead. In doing so, we have attempted to place the various contributions of this special volume within a broader context. 2. The status and base of the Quaternary System/Period (2.58–0 Ma) 2.1. Initial phase: 1759–1854 The Quaternary was first introduced by the mining engineer Giovanni Arduino in 1759 (Arduino 1760), as a “fourth order” (quarto ordine) in his fourfold (chrono- )lithostratigraphic subdivision of geological time, based on his observations of the Venetian and Tuscan regions of Italy. This “fourth order” comprised alluvial deposits, some of which were stratified, that he observed on the Po plain. Although he never actually used the term “quaternario”, this concept has been used for 255 years (Vaccari, 2006; Head et al., 2008a). It was Desnoyers (1829, p. 193) who applied the term Quaternaire or tertiaires récens to strata of the Loire, Gironde, Hérault, and Rhône basins and in Italy that were younger than the “older Tertiary” of the Seine (Paris) Basin (Bourdier, 1957, p. 99; Farrand, 1990; Berggren, 1998), although he used this term only for brevity and ironically waived it in the same publication because of its transitional nature with the “older Tertiary” (footnote to p. 193 in Desnoyers, 1829). De Serres (1830) considered the Quaternaire synonymous with the term Diluvium, and noted that humans were contemporaneous with these deposits. It was finally defined on paleontological grounds in 1833 by H. Reboul who remarked “This is why one had to call quaternary, the subsequent period, which is characterised by plant and animal species similar to those that occur in the same places today” (Reboul, 1833, p. 1, 2). Reboul credited Desnoyers as being first to use the term Quaternaire, which he used 3 unequivocally to distinguish from the Tertiary. The extent of the Quaternary was vaguely and variably applied in these early usages (Berggren, 1998). In the early 19th century, prompted by Louis Agassiz’s then new Glacial Theory (Agassiz, 1840, 1841), widespread erratics in Alpine and northern Europe were seen as having been deposited by extensive ice; and deposits comprising sands, gravels and boulder clays (previously labeled as “Diluvium” under the belief that they were the product of a Biblical flood or deluge) were attributed to deposition from floating ice and hence called “Drift” (Harland et al., 1982). Consequently, the Quaternary became synonymous with what Goethe had termed the Ice Age (Pillans and Gibbard, 2012), and indeed Morlot (1854/56) defined and subdivided his Quaternary on the basis of glaciation.